Exhaust gas cleaning device

ABSTRACT

An exhaust cleaning device includes a NOx storage reduction catalyst storing NOx when an air-fuel ratio of an inflowing exhaust gas is lean and releasing the stored NOx when the air-fuel ratio of the inflowing exhaust gas is rich; an air-fuel ratio changing unit; a reducing agent supplying unit which adds a reducing agent to the NOx storage reduction catalyst; a temperature detection unit which detects a temperature of the NOx storage reduction catalyst; a poisoning recovery timing judging unit which judges a timing for executing a SOx poisoning recovery process of releasing SOx from the NOx storage reduction catalyst; and a control unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns an exhaust gas cleaning device for eliminating hazardous components and microparticles, etc., that are contained in the exhaust gas of an internal combustion engine.

2. Description of the Related Art

With a lean-combustion-enabled internal combustion engine, such as a diesel engine, in which engine operation is performed by combustion of a mixed gas of high air-fuel ratio (lean atmosphere) within a wide operation range, a NOx catalyst, having the function of cleaning the exhaust gas of nitrogen oxides (NOx), is equipped in an exhaust passage of the engine. As the NOx catalyst, for example, a catalyst, wherein a NOx storage agent, having the ability to store NOx under the presence of oxygen, and a noble metal catalyst, having the ability to oxidize hydrocarbons (HC), are carried in combination by a carrier, formed of porous ceramic, etc., and having a honeycomb structure, is employed.

A NOx catalyst has a property of storing NOx when the air-fuel ratio of the exhaust is no less than a theoretical air-fuel ratio (this state shall be referred to hereinafter as the “lean”state) and releasing NOx when the air-fuel ratio of the exhaust is no more than the theoretical air-fuel ratio (this state shall be referred to hereinafter as the “rich”state). If when NOx is released into an exhaust gas, HC, CO, etc., exist in the exhaust gas, redox reactions, in which NOx acts as an oxidizing component and HC and CO act as reducing components, occur among these components due to oxidation reactions of HC and CO being promoted by the noble metal catalyst. HC and CO are thus oxidized to CO2 and H2O and NOx is reduced to N2.

Even when the exhaust gas is in the lean state, if a predetermined limiting amount of NOx is stored by the NOx catalyst, the NOx catalyst will not store any more NOx. Thus generally, before the NOx storage amount of the NOx catalyst reaches the limiting amount, a recovery operation control, by which, light oil or other reducing agent, used as fuel at the upstream side of the NOx catalyst in an exhaust passage, is supplied by a reducing agent supplying unit to release and carry out reduction cleaning of the NOx stored in the NOx catalyst to thereby revive the NOx storing capacity of the NOx catalyst, is repeated at predetermined intervals.

However, since sulfur components are contained in the fuel (light oil) of an internal combustion engine, sulfur oxides (SOx), originating from such sulfur components in the fuel, exist along with NOx in the exhaust gas. Since the SOx existing in the exhaust gas is stored by the NOx catalyst and moreover is not readily released from the catalyst even under conditions that are adequate for releasing the NOx stored in the catalyst (exhaust gas rich conditions), the SOx in the exhaust gas becomes gradually stored in the NOx catalyst to cause S poisoning as engine operation is continued.

As a method of efficiently decomposing and eliminating microparticles and the SOx that have become stored in the NOx catalyst, there is known an operation control (referred to hereinafter as S poisoning recovery control), wherein the fuel injection amount into an engine is increased while decreasing the air intake amount to make the air-fuel ratio of the exhaust denser and somewhat richer than the theoretical air-fuel ratio (stoichiometric ratio) and adding fuel to the exhaust passage upstream the NOx catalyst by the reducing agent supplying unit to raise the temperature of the NOx catalyst to a target temperature of, for example, 600° C. or higher. By executing such S poisoning recovery control, the reducing components in the exhaust gas that has been adjusted to the stoichiometric ratio or somewhat richer decompose and eliminate the SOx stored in the catalyst under the high temperature condition. However, since, even while the reducing components in the exhaust gas are eliminating the SOx, etc., that have become stored in the catalyst, the catalyst continues to be heated by the reaction heat of the reducing components, the catalyst temperature may undergo an excessive temperature rise and exceed a limiting temperature of the catalyst or the catalyst carrier.

Also with the NOx storage catalyst, though NOx is stored during lean air-fuel ratio control, if lean fuel combustion operation is continued over a long period of time, since there is a limit to the NOx storage amount of the catalyst, the NOx in the exhaust gas stops becoming stored in the catalyst and becomes discharged to the atmosphere at the point at which the NOx storage amount reaches saturation. Thus before the storage amount of the NOx storage catalyst reaches saturation, an operation called a rich spike is executed wherein switching to rich air-fuel ratio operation, in which the air-fuel ratio is controlled to be no more than the theoretical air-fuel ratio, is carried out periodically.

JP-A-2001-304011 describes the setting of the exhaust air-fuel ratio gradually towards the rich side when the temperature of a NOx storage catalyst is no less than a predetermined temperature.

Though a NOx storage catalyst can store NOx even at a temperature less than or equal to a reduction enabling temperature, if the catalyst temperature is raised in the process of performing S poisoning recovery control on the NOx storage catalyst, the stored NOx may become released due to thermal dissociation and NOx slip, in which the NOx becomes discharged without being cleaned, may occur in the transition to S poisoning recovery control. The temperature at which NOx becomes stored in the NOx storage catalyst ranges widely from a low exhaust gas temperature to a high exhaust gas temperature, and the above phenomenon is presumed to occur in particular due to the release of the NOx that was stored at low temperatures.

Though JP-A-2001-304011 describes the setting of the exhaust air-fuel ratio gradually towards the rich side when the temperature of the NOx storage catalyst is no less than the predetermined temperature, the thermal dissociation temperature of the catalyst is not taken into consideration.

On the other hand, JP-A-2003-120373 describes the addition of an additive across a plurality of times in raising the temperature of the NOx storage catalyst.

Though a NOx storage catalyst can perform storage even at a temperature no more than a reduction enabling temperature, when the catalyst temperature is raised in the process of performing S poisoning recovery control on the NOx storage catalyst, the stored NOx becomes released due to thermal dissociation before the catalyst activation temperature is reached and NOx slip, in which the NOx becomes discharged without becoming cleaned, may occur in the transition to S poisoning recovery control. Also, if the temperature is raised at once to the S poisoning recovery enabling temperature, the catalyst temperature may undergo an excessive temperature rise and exceed the limiting temperature of the catalyst or the catalyst carrier.

Though JP-A-2003-120373 describes the adding of an additive across a plurality of times during the raising of the temperature of a NOx storage catalyst, it does not take into consideration a reaction enabling temperature of the catalyst.

SUMMARY OF THE INVENTION

An object of this invention is to provide an exhaust gas cleaning device, which, during S poisoning recovery control, can prevent sudden temperature rises of the NOx storage catalyst to restrain NOx slip due to thermal dissociation.

According to a first aspect of the invention, an exhaust cleaning device includes: a NOx storage reduction catalyst, being disposed in an exhaust passage of a lean-combustion-enabled internal combustion engine and, the Nox storage reduction catalyst storing NOx when an air-fuel ratio of an inflowing exhaust gas is lean and releasing the stored NOx when the air-fuel ratio of the inflowing exhaust gas is rich; an air-fuel ratio changing unit which changes the air-fuel ratio; a reducing agent supplying unit which adds a reducing agent to the NOx storage reduction catalyst; a temperature detection unit which detects a temperature of the NOx storage reduction catalyst; a poisoning recovery timing judging unit which judges a timing for executing a SOx poisoning recovery process of releasing SOx from the NOx storage reduction catalyst; and a control unit making the reducing agent be added from the reducing agent supplying unit when the poisoning recovery timing judging unit judges that the timing for executing the SOx poisoning recovery process has arrived, and the control unit controlling the air-fuel ratio changing unit to put the air-fuel ratio in a rich state if the detected temperature from the temperature detection unit is lower than a thermal dissociation temperature of the NOx storage reduction catalyst and is higher than a NOx reduction reaction enabling temperature.

With the present invention, when the poisoning recovery timing judging unit judges that the timing for process execution has arrived, the added amount of the reducing agent, added from the reducing agent supplying unit, is made to increase gradually. The rate of temperature rise of the catalyst will thus be gradual. Also, if during the increase of the added amount of the reducing agent, the detected temperature from the temperature detection unit is lower than the thermal dissociation temperature of the NOx storage reduction catalyst, the air-fuel ratio changing unit is controlled to put the air-fuel ratio in a rich state. The NOx stored in the NOx storage reduction catalyst can thus be reduced before the NOx storage reduction catalyst reaches the thermal dissociation temperature, thereby lessening the amount of NOx slip that occurs when the thermal dissociation temperature is reached. Also, since the rate of temperature rise of the catalyst is made gradual, excessive rise of the temperature (excessive temperature rise) of the NOx storage reduction catalyst can be restrained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of the arrangement of an exhaust gas cleaning device of an embodiment of this invention and an engine onto which this device is installed;

FIG. 2 shows line diagrams of the relationships of the poisoning recovery period, reducing agent supply amount, catalyst temperature, and rich spike execution timing in a first embodiment of the invention;

FIG. 3 shows a flowchart of an embodiment of control by a control unit in a first embodiment of the invention.

FIG. 4 shows line diagrams of the relationships of the poisoning recovery period, reducing agent supply amount, catalyst temperature, and rich spike execution timing in a first embodiment of the invention; and

FIG. 5 shows a flowchart of an embodiment of control by a control unit in a first embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFFERED EMBODIMENTS First Embodiment

FIG. 1 shows an exhaust gas cleaning device, which is an embodiment of this invention. This exhaust gas cleaning device is applied to an internal combustion engine (referred to hereinafter as “engine”) 1. The engine 1 is a four-cylinder engine arranged with a fuel supply system 2, combustion chambers 3, an air intake system 4, an exhaust system 5, etc., as the principal parts. The fuel supply system 2 is equipped with a supply pump 9, a common rail 6, and injectors 7, which are the main injection unit and function as air-fuel ratio changing unit. The supply pump 9 is driven by the engine 1, applies high pressure to fuel pumped up from an unillustrated fuel tank, and supplies the fuel to the common rail 6 via an engine fuel passage 8. The common rail 6 functions as a pressure accumulating chamber that holds (accumulates the pressure of) the high-pressure fuel, supplied from the supply pump 9, at a predetermined pressure, and distributes the pressure-accumulated fuel among the injectors 7 that are disposed so as to face the combustion chambers 3 of the respective cylinders.

Each injector 7 is a known type of solenoid valve that is equipped in its interior with an illustrated electromagnetic solenoid. Each injector 7 supplies fuel directly into a corresponding cylinder based on the engine operation state in either the intake processor the compression process of the engine 1 and the basic injection amount for obtaining a driving force is determined by an unillustrated map.

The air intake system 4 forms a passage (intake air passage) for the intake air supplied into the respective combustion chambers 3, and the exhaust system 5 forms a passage (exhaust passage) for the exhaust gas discharged from the respective combustion chambers 3. The engine 1 is provided with a turbocharger 10, which is a known type of turbocharger. The turbocharger 10 is equipped with rotators 12 and 13, which are connected via a shaft 11. One of the rotators is a turbine wheel 12, which is exposed to the exhaust inside the exhaust system 5, and the other rotator is a compressor wheel 13, which is exposed to the intake air inside the air intake system 4. The turbocharger 10, with such an arrangement, performs a known form of turbocharging by using the exhaust flow (exhaust pressure), received by the turbine wheel 12, to rotate the compressor wheel 13 and increase the air intake pressure. At the upstream side of air intake with respect to the compressor wheel 13 is disposed an air flow sensor 28, which outputs a detection signal in accordance with a flow rate (intake rate) of the air (intake air) that is introduced into the air intake system 4.

An intercooler 14, which is disposed in the air intake system 4 at the downstream side of the turbocharger 10, forcibly cools the intake air that has been raised in temperature by the turbocharging. A throttle valve 15, which is disposed further downstream from the intercooler 14 and functions as an air-fuel ratio changing unit, is an electronically operated opening/closing valve, with which the opening can be adjusted in a stage-less manner, and has the function of adjusting the supply rate (flow rate) of the intake air by changing the flow path area for the intake air under predetermined conditions.

The engine 1 has formed therein an EGR passage 16, which serves as an exhaust reflow passage that bypasses the upstream sides (air intake system 4) and the downstream sides (exhaust system 5) of the combustion chambers 3. The EGR passage 16 has the function of returning a part of the exhaust to the air intake system 4 when suitable. In the EGR passage 16 are disposed an EGR valve 17, which is opened and closed in a stage-less manner by electronic control, enables the flow rate of the exhaust gas (EGR gas) flowing through the passage to be adjusted freely, and also functions as an air-fuel ratio changing unit, and an EGR cooler 18, which cools the exhaust that passes (reflows) through the EGR passage 16.

In the exhaust passage 50 at the downstream side in the direction of exhaust gas outflow from the turbine wheel 12 are disposed a reducing agent injection nozzle 21, which supplies and adds fuel that serves as a reducing agent into the exhaust gas that flows through the exhaust passage 50, and a NOx storage reduction catalyst (referred to hereinafter as “NOx catalyst”) 20, which is housed inside a casing 27 and cleans the exhaust gas.

The reducing agent injection nozzle 21 is connected via an added fuel passage 23 to the supply pump 9 and is thereby supplied with a part of the fuel that is pumped up from the fuel tank. In the added fuel passage 23 is disposed a solenoid regulating valve 24, which adjusts the flow rate of fuel from the supply pump 9 to the reducing agent injection nozzle 21 and opens and closes the added fuel passage 23. The solenoid regulating valve 24 is controlled in the timing of opening/closing by a control unit 25 and supplies and adds the fuel that serves as the reducing agent to the exhaust gas in the exhaust passage 50 at the upstream side of the NOx catalyst 20.

An air-fuel ratio (A/F) sensor 30 is disposed in the exhaust passage 50 at the upstream side of the NOx catalyst 20. The air-fuel ratio (A/F) sensor 30 outputs a detection signal that varies continuously in accordance with an oxygen concentration in the exhaust gas at the upstream side of the casing 27. In the exhaust passage 50 at the downstream side of the NOx catalyst 20 are disposed an exhaust temperature sensor 32, which serves as a temperature detection unit that detects a temperature of the NOx catalyst 20, and a NOx sensor 31. The NOx sensor 31 outputs a detection signal that varies continuously in accordance with a NOx concentration in the exhaust gas at the downstream side of the NOx catalyst 20. An accelerator position sensor 33 is mounted to an unillustrated accelerator pedal of the engine 1 and outputs a detection signal that is in accordance with an amount by which the pedal is depressed. A crank angle sensor 34 outputs a detection signal (pulse) each time an output shaft (crankshaft) of the engine 1 rotates by a fixed angle. These sensors 30 to 34 are electrically connected to the input side of the control device 25.

The control unit 25 is arranged from a known computer, equipped with a CPU, ROM, RAM, backup RAM, timer counter, etc. The control unit 25 inputs the detection signals of various sensors via an unillustrated external input circuit and, based on these signals, performs control concerning the valve opening/closing operations of the injectors 7, opening adjustment of the EGR valve 17, opening adjustment of the throttle valve 15, and various other controls concerning the operation state of the engine 1.

The NOx catalyst 20 is housed in the casing 27 and installed on the exhaust passage 50. The NOx catalyst 20 has a carrier, which is honeycomb-shaped structure, and on the surface of the carrier, an alkali metal, such as potassium (K), sodium (Na), lithium (Li), or cesium (Cs), an alkaline earth, such as barium (Ba) or calcium (Ca), or a rare earth, such as lanthanum (La) or yttrium (Y), is carried to function as a NOx storing agent and a noble metal, such as platinum (Pt), is carried to function as an oxidation catalyst (noble metal catalyst).

A NOx storage agent has a property of storing NOx when the exhaust gas is in the lean state and releasing the NOx in the rich state. If when the NOx is released into the exhaust gas, HC, CO, etc., exist in the exhaust gas, redox reactions, in which NOx acts as oxidizing components and HC and CO act as reducing components, occur among these components due to oxidation reactions of HC and CO being promoted by the noble metal catalyst. The HC and CO are thus oxidized to CO2 and H2O and the NOx is reduced to N2.

Since, after a predetermined limiting amount of NOx has been stored, a NOx storage agent becomes unable to store more NOx even when the exhaust gas is in the lean state, the stored NOx must be reduced to N2, etc., in a reducing atmosphere and eliminated as described above. Thus with the present exhaust gas cleaning device, a so-called rich spike, wherein rich air-fuel ratio operation is performed by decreasing the air intake amount by the EGR or the air intake throttle, increasing the engine injection amount, and performing fuel injection into the exhaust pipe, is executed at a priorly determined cycle and for a predetermined length of time to forcibly create a CO-excessive state, in other words, a reducing atmosphere inside the NOx storage catalyst 20 and thereby release, reduce, and eliminate the stored NOx (NOx purge). In actuality, the above-mentioned predetermined cycle is timed by a timer counter inside the ECU 25, and the EGR valve 17, throttle valve 15, injectors 7, and reducing agent injection nozzle 21 are controlled by the ECU 25. Though the predetermined cycle is set based on a time by which the NOx stored in the NOx storage catalyst 20 is presumed to reach the saturation amount under normal engine operation, the cycle can also be estimated, for example, on the basis of a travel distance of a vehicle. That is, the rich fuel-air ratio operation (rich spike) may be executed after traveling for a predetermined distance has been carried out.

The ECU 25 performs control of fuel injection by injectors 7 based on the operation state of the engine 1 that is ascertained by the detection signals of the various sensors. Here, fuel injection control concerns the injection of fuel into the respective combustion chambers 3 via the corresponding injectors 7 and is a series of processes wherein such parameters as the fuel injection amount, injection timing, and injection pattern are set and operation of the opening/closing valves of the respective injectors 7 are executed based on these set parameters. The ECU 25 performs such a series of processes repeatedly within a predetermined time during the operation of the engine 1. The fuel injection amount and the injection timing are determined based on the amount of depression of the accelerator pedal and an engine rotation speed (a parameter that can be computed based on the pulse signal from the crank angle sensor) and referencing an unillustrated map that has been set priorly.

In regard to the setting of the fuel injection pattern, the ECU 25 obtains engine output by performing fuel injection near the compression top dead center as the main injection into the respective cylinders, and also performs fuel injection (referred to hereinafter as “pilot injection”) prior to the main injection and fuel injection (referred to hereinafter as “post injection”) following the main injection as auxiliary injections on selected cylinders at suitably selected timings.

The fuel that is injected into a combustion chamber 3 by post injection is modified to light HC in the combustion gas and discharged to the exhaust system 5. That is, light HC, which functions as a reducing agent, is added to the exhaust system 5 via the post injection and the reducing component concentration in the exhaust gas is thereby increased. The reducing components added to the exhaust system reacts, via the NOx catalyst 20, with the NOx released from the NOx catalyst and other oxidizing components contained in the exhaust gas. The reaction heat that is generated in this process raises the bed temperature (temperature) of the NOx catalyst. As the rich air-fuel ratio operation (rich spike), post injection may be executed instead of the control of increasing the injection amount of main injection by the injectors 7 at every predetermined cycle.

The ECU 25 performs EGR control based on the operation conditions of the engine 1 that are ascertained from the detection signals of the various sensors. EGR control refers to the driving and operation of the EGR valve 17 disposed in the EGR passage 16 for adjustment of the flow rate of the gas passing through the EGR passage 16, that is, the flow rate of the exhaust gas that is recycled from the exhaust system 5 to the air intake system 4.

The valve opening amount of the EGR valve 17 that is targeted (referred to hereinafter as the “target valve opening amount”) is basically determined based on operation conditions, such as the load, rotation speed, etc., of the engine 1, and is determined in reference to an unillustrated map that is determined priorly. The ECU 25 renews this target valve opening amount at every predetermined timing during the operation of the engine 1 and successively outputs command signals to a drive circuit of the EGR valve 17 so that the actual valve opening amount of the EGR valve 17 matches the renewed target valve opening amount.

As with post injection, the reducing component concentration in the exhaust gas can be increased and the bed temperature of the NOx catalyst 20 can consequently be raised by directly adding the fuel (reducing agent) to the exhaust system 5 via the reducing agent injection nozzle 21. In comparison to the fuel added by post injection, the fuel that is added by the reducing agent injection nozzle 21 tends to be maintained more in the macromolecular state and be distributed non-uniformly in the exhaust gas. Also, with fuel addition by the reducing agent injection nozzle 21, the degrees of freedom in regard to the amount of fuel that can be added at once and the timing of addition are greater than those in the case of post injection.

The S poisoning recovery control shall now be described in outline. Since the above-described post injection and fuel addition control both act to increase the reducing components in the exhaust gas, by repeatedly performing either control at predetermined intervals, the NOx stored in the NOx catalyst 20 can be released and subject to reduction cleaning to recover the NOx storing capacity of the NOx catalyst 20.

In order to eliminate the SOx, etc., that becomes gradually stored in the NOx catalyst as engine operation of the engine 1 is continued, the ECU 25 performs enrichment control (referred to hereinafter as “S poisoning recovery control”), wherein the air-fuel ratio prior to the catalyst is set to no more than a theoretical air-fuel ratio upon raising the temperature of the NOx catalyst 20 to no less than a target temperature (for example, approximately 600° C.) as shown in FIG. 2. By performing the S poisoning recovery control, the large amount of the reducing components supplied to the NOx catalyst 20 eliminates, under the high-temperature condition, the SOx stored in the catalyst. As part of the S poisoning recovery control, the ECU 25 performs the above-mentioned post injection or exhaust fuel addition control to raise the temperature of the NOx catalyst 20 to the target temperature. In the present embodiment, a control (referred to hereinafter as “reducing component supply control”), whereby a larger amount of fuel (reducing components) than is required for release and reduction cleaning of the NOx stored in the NOx catalyst 20 is supplied to the upstream side of the NOx catalyst 20 via the reducing agent injection nozzle 21, is performed.

In the S poisoning recovery control, a large amount of reducing components is supplied to the upstream side of the NOx catalyst in the exhaust system upon establishing the condition of maintaining the temperature of the NOx catalyst 20 at the S purge target temperature (600° C.) necessary for S poisoning recovery as shown in FIG. 2. However, though the large amount of reducing components supplied into the exhaust system exhibits the function of eliminating the SOx, etc., stored in the NOx catalyst 20 under the high-temperature condition, it also has the characteristic of raising the temperature of the NOx catalyst 20 further. Thus when the large amount of reducing components is supplied continuously to the upstream side of the NOx catalyst in the exhaust system under normal operation conditions, the NOx catalyst 20 may become heated excessively and the stored NOx may become released due to thermal dissociation.

The ECU 25 thus has a poisoning recovery timing judging unit 42 and, when it is judged by the poisoning recovery timing judging unit 42 that the timing for execution of the recovery process has arrived, makes the added amount of the reducing agent, added from the reducing agent injection nozzle 21, increase gradually and, if the detected temperature from the exhaust temperature sensor 32 is a predetermined temperature (FIG. 2) that is less than the thermal dissociation temperature of the NOx catalyst 20, controls the EGR valve 17, throttle valve 15, injectors 7, and reducing agent injection nozzle 21 to execute a rich spike that puts the air-fuel ratio into the rich state. That is, if the predetermined cycle of the rich spike arrives with the temperature being lower than the thermal dissociation temperature, the rich spike is executed, while if the state in which temperature is lower than the thermal dissociation temperature is shifted in time with respect to the predetermined timing, the predetermined cycle is ignored and the rich spike is executed forcibly while the temperature is lower than the thermal dissociation temperature.

The poisoning recovery timing judging unit 42 judges whether or not S poisoning of the NOx catalyst 20 is progressing. In the present embodiment, the poisoning recovery timing judging unit 42 judges the recovery timing from the history of detection signals from the NOx sensor 31 and if it recognizes that the NOx cleaning function of the NOx catalyst 20 is degrading, it outputs an S purge signal, which signals the timing of execution of the S poisoning recovery control.

The added amount of the reducing agent that is added from the reducing agent injection nozzle 21 is determined priorly by a test and mapped so that it increases with the elapse of time from the point at which the S purge signal is output as shown in FIG. 2.

The S poisoning recovery control of the exhaust gas cleaning device with such an arrangement shall now be described in line with the flowchart shown in FIG. 5. In step S1, whether or not the S purge signal is output is judged, and if this signal is output, step S2 is entered, the solenoid regulating valve 24 is controlled by the ECU 25, and the fuel that is to serve as the reducing agent is added into the exhaust gas by being injected by the reducing agent injection nozzle 21 with the fuel injection amount (addition amount) being set lower than normal. In the exhaust system 5, the temperature of the NOx catalyst 20 rises gradually due to the added fuel as shown in FIG. 2. The ECU 25 takes in the signal from the temperature sensor 32, and if in step S3, the detected catalyst temperature is no less than the NOx reduction reaction enabling temperature and is less than a predetermined temperature (a temperature that is lower than the thermal dissociation temperature by a predetermined amount), step S4 is entered and a rich spike is executed.

Thus when the S purge signal, which signals the timing of execution of poisoning recovery, is output from the poisoning recovery timing judging unit 42, since opening/closing control of the solenoid regulating valve 24 is performed to gradually increase the added amount of the reducing agent added from the reducing agent injection nozzle 21, the temperature rise of the catalyst will be gradual. Also, since if, during increase of the added amount of the reducing agent, the detected temperature from the exhaust temperature sensor 32 is a predetermined temperature lower than the thermal dissociation temperature of the NOx catalyst 20, the air-fuel ratio is put in the rich state, the NOx stored in the NOx catalyst 20 can be reduced before the NOx catalyst 20 reaches the thermal dissociation temperature. The amount of NOx slip, which occurs when the temperature of the NOx catalyst 20 rises due to the exhaust gas and reaches the thermal dissociation temperature, can thus be lessened. Also, since the rate of temperature rise of the catalyst will be gradual, excessive rise of the temperature (excessive temperature rise) of the NOx catalyst 20 can be restrained to thereby reduce thermal degradation and improve durability.

In FIG. 2, the alternate long and two short dashes lines indicate the NOx slip amount and the temperature rise of the NOx catalyst 20 when the additive is injected from the reducing agent injection nozzle 21 by an amount that raises the temperature of the NOx catalyst 20 to the S purge target temperature at once, and the solid lines indicate the NOx slip amount and the catalyst temperature rise characteristics when, as in the present embodiment, the amount of additive from the reducing agent injection nozzle 21 is increased gradually and a rich spike is performed once prior to the reaching of the thermal dissociation temperature. As is clear from FIG. 2, when a rich spike is executed at a predetermined temperature that is lower than the thermal dissociation temperature, the NOx slip amount decreases drastically.

Second Embodiment

Newt, second embodiment of the invention will be described below. In this second embodiment, as shown in FIG. 4, in the S poisoning recovery control, a large amount of reducing components is supplied to the upstream side of the NOx catalyst in the exhaust system upon establishing the condition of maintaining the temperature of the NOx catalyst 20 at the S purge target temperature (600° C.) necessary for S poisoning recovery. However, though the large amount of reducing components supplied into the exhaust system exhibits the function of eliminating the SOx, etc., stored in the NOx catalyst 20 under the high-temperature condition, it also has the characteristic of raising the temperature of the NOx catalyst 20 further. Thus when the large amount of reducing components is supplied continuously to the upstream side of the NOx catalyst in the exhaust system, the NOx catalyst 20 may undergo excessive temperature rise, the stored NOx may become released due to thermal dissociation, and, if the additive (HC) is added into the exhaust gas before the reaction-enabling temperature (HC light-off temperature), at which the NOx catalyst 20 exhibits at least its minimum function, is reached, uncombusted HC may become released into the atmosphere or the HC may become adsorbed onto the catalyst and this HC may undergo combustion at once after the catalyst temperature reaches the catalyst reaction enabling temperature and thereby cause an excessive temperature rise.

The ECU 25 thus has a poisoning recovery timing judging unit 42 and, when it is judged by the poisoning recovery timing judging unit 42 that the timing for execution of the recovery process has arrived, performs a first reducing agent addition (first addition), indicated by the symbol A in FIG. 4, by reducing agent injection nozzle 1 so that the detected temperature t0 from the exhaust temperature sensor 32 will be no less than the reduction reaction enabling temperature t2 of the NOx catalyst 20 and yet less than the thermal dissociation temperature t4 and, when the detected temperature t0 from the exhaust temperature sensor 32 exceeds the reduction reaction enabling temperature t2, executes a rich spike to put the air-fuel ratio in the rich state and, after execution of this rich spike, controls the reducing agent injection nozzle 21 to perform a second reducing agent addition (second addition), indicated by the symbol B in FIG. 4, so that the detected temperature t0 from the exhaust temperature sensor 32 will become equal to the SOx poisoning recovery target temperature t3 of the NOx catalyst 20. In performing the rich spike, the EGR valve 17, throttle valve 15, injectors 7, and reducing agent injection nozzle 21 are controlled.

Also, when the detected temperature t0 from the exhaust temperature sensor 32 is less than the HC light-off temperature t1, which is the catalyst reaction enabling temperature, the ECU 25 performs temperature raising control of the engine 1 so that the detected temperature t0 from the exhaust temperature sensor 32 will become equal to the HC light-off temperature t1.

The poisoning recovery timing judging unit 42 judges whether or not S poisoning of the NOx catalyst 20 is progressing. In the present embodiment, the poisoning recovery timing judging unit 42 judges the recovery timing from the history of detection signals from the NOx sensor 31 and when it recognizes that the NOx cleaning function of the NOx catalyst 20 is degrading, it outputs an S purge signal, which signals the timing of execution of S poisoning recovery control.

The added amount of the reducing agent, added from the reducing agent injection nozzle 21, is determined priorly by a test and mapped so that in the injection in the first addition, the catalyst temperature will be no less than the NOx reduction enabling temperature and yet less than the NOx thermal dissociation temperature and so that in the injection in the second addition after execution of the rich spike, the additive amount will be such as to enable the catalyst temperature to reach the SOx poisoning recovery target temperature as shown in FIG. 4. The respective information of the HC light-off temperature t1, reduction reaction enabling temperature t2, and SOx poisoning recovery temperature t3 are stored and set in the ROM of the ECU 25. In the present embodiment, the reaction enabling temperature t1 is assumed to be 200° C., the reduction reaction enabling temperature t2 is assumed to be 250° C., which is higher than the HC light-off temperature t1 and lower than the thermal dissociation temperature t4, and the SOx poisoning recovery target temperature t3 is assumed to be 600° C.

With the present embodiment, as the temperature raising control by the engine 1, the exhaust temperature is raised by controlling the engine combustion by increasing the idling rotation speed, closing the intake throttle, etc.

The S poisoning recovery control of the exhaust gas cleaning device with such an arrangement shall now be described in line with the flowchart shown in FIG. 5. In step S1, whether or not the detection signal (catalyst temperature t0) from the exhaust temperature sensor 32 is higher than the catalyst reaction enabling temperature t1 is judged, and if the catalyst temperature t0 is less than the catalyst reaction enabling temperature t1, step S2 is entered and the engine exhaust temperature raising control is executed. When by this control, the NOx catalyst 20 is heated and the catalyst temperature t0 exceeds the catalyst reaction enabling temperature (HC light-off temperature) t1, step S3 is entered. In step S3, whether or not the S purge signal is output is judged, and if this signal is output, step S4 is entered, the solenoid regulating valve 24 is controlled by the ECU 25, and the reducing agent of the first addition is injected into the exhaust gas from the reducing agent injection nozzle 21. By this injection, combustion occurs in the NOx catalyst 20 and the catalyst temperature is raised and the temperature distribution of the NOx catalyst 20 that is long in the exhaust gas flow path direction is made uniform at a temperature less than the thermal dissociation temperature.

In steps S5, whether or not the catalyst temperature t0 has reached the reduction reaction enabling temperature t2 is judged and if the reduction reaction enabling temperature t2 is reached, step S6 is entered to execute a rich spike by control of the EGR valve 17, throttle valve 15, and reducing agent injection nozzle 21 and then step S7 is entered. When the rich spike is executed in step S6, since the reduction reaction enabling temperature t2 is no more than the thermal dissociation temperature t4, the NOx stored in the NOx catalyst 20 can be lessened before the NOx catalyst 20 reaches the thermal dissociation temperature.

In step S7, the ECU 25 controls the solenoid regulating valve 24 to make the reducing agent of the second addition be injected from the reducing agent injection nozzle 21 into the exhaust gas in the exhaust passage 50, and in step S8, the reducing agent is added until the catalyst temperature t0 becomes equal to the SOx poisoning recovery target temperature t3. By injection of the reducing agent, adequate combustion occurs in the NOx catalyst 20 and since the temperature is raised to the target temperature t3, the SOx stored in the NOx catalyst 20 is eliminated.

Since, at the timing of execution of the SOx poisoning recovery process of releasing SOx from the NOx catalyst 20, the first reducing agent addition is performed by the reducing agent injection nozzle 21 so that the detected temperature t0 from the exhaust temperature sensor 32 will be the reduction reaction enabling temperature t2 that is no less than the catalyst reaction enabling temperature t1 of the NOx catalyst 20 and yet less than the thermal dissociation temperature t4 and, when the reduction reaction enabling temperature t2 is exceeded, the air-fuel ratio is put in the rich state, the NOx stored in the NOx catalyst 20 can be reduced efficiently and NOx slip due to thermal dissociation can be lessened. Also when the second reducing agent addition is performed by the reducing agent injection nozzle 21 so that the detected temperature t0 from the exhaust temperature sensor 32 will become equal to the SOx poisoning recovery target temperature t3 of the NOx storage reduction catalyst after the air-fuel ratio control, the catalyst temperature is raised to the SOx poisoning recovery target temperature t3 from the state in which the temperature distribution of the catalyst has been stabilized and the SOx poisoning recovery process can thus be performed while restraining excessive temperature rise of the NOx catalyst 20.

Furthermore with the present embodiment, since when the catalyst temperature t0 is less than the catalyst reaction enabling temperature t1, temperature raising by combustion control of the engine 1 is performed, the amount of HC that becomes adsorbed onto the catalyst can be lessened and the slip amount of uncombusted HC can be lessened in comparison to the case where the reducing agent is added directly into the exhaust passage 50.

In FIG. 4, the alternate long and two short dashes lines indicate the NOx slip amount and the temperature rise of the NOx catalyst when the additive is injected from the reducing agent injection nozzle 21 by an amount that raises the temperature of the NOx catalyst 20 to the S purge target temperature at once, and the solid lines indicate the NOx slip amount and the catalyst temperature rise characteristics when, as in the present embodiment, the additive is injected from the reducing agent injection nozzle 21 in two stages and a rich spike is executed between the first addition and the second addition. As is clear from FIG. 4, when a rich spike is executed in the state in which the catalyst temperature has risen to the reduction reaction enabling temperature t2, which is lower than the thermal dissociation temperature, by the first addition, the NOx slip amount decreases drastically. 

1. An exhaust cleaning device comprising: a NOx storage reduction catalyst, being disposed in an exhaust passage of a lean-combustion-enabled internal combustion engine and, the Nox storage reduction catalyst storing NOx when an air-fuel ratio of an inflowing exhaust gas is lean and releasing the stored NOx when the air-fuel ratio of the inflowing exhaust gas is rich; an air-fuel ratio changing unit which changes the air-fuel ratio; a reducing agent supplying unit which adds a reducing agent to the NOx storage reduction catalyst; a temperature detection unit which detects a temperature of the NOx storage reduction catalyst; a poisoning recovery timing judging unit which judges a timing for executing a SOx poisoning recovery process of releasing SOx from the NOx storage reduction catalyst; and a control unit making the reducing agent be added from the reducing agent supplying unit when the poisoning recovery timing judging unit judges that the timing for executing the SOx poisoning recovery process has arrived, and the control unit controlling the air-fuel ratio changing unit to put the air-fuel ratio in a rich state if the detected temperature from the temperature detection unit is lower than a thermal dissociation temperature of the NOx storage reduction catalyst and is higher than a NOx reduction reaction enabling temperature.
 2. The exhaust gas cleaning device according to claim 1, wherein, when the poisoning recovery timing judging unit judges that the timing for executing the SOx poisoning recovery process has arrived, the control unit makes the added amount of the reducing agent which is added from the reducing agent supplying unit, increase gradually and, if the detected temperature from the temperature detection unit is lower than the thermal dissociation temperature of the NOx storage reduction catalyst and is higher than the NOx reduction reaction enabling temperature, the control unit controls the air-fuel ratio changing unit to put the air-fuel ratio in a rich state.
 3. The exhaust gas cleaning device according to claim 1, further comprising a control unit, which, in the SOx poisoning recovery process of releasing SOx from the NOx storage reduction catalyst, controls the air-fuel ratio changing unit and the reducing agent supplying unit to perform a first reducing agent addition by the reducing agent supplying unit so that the detected temperature from the temperature detection unit become a temperature in a range that is higher than the reduction reaction enabling temperature of the NOx storage reduction catalyst and lower than the thermal dissociation temperature of the NOx storage reduction catalyst and, when the detected temperature from the temperature detection unit becomes a temperature in the range, sets the air-fuel ratio to a rich state by the air-fuel ratio changing unit and executes a second reducing agent addition by the reducing agent supplying unit so that the detected temperature from the temperature detection unit becomes equal to a SOx poisoning recovery target temperature for the NOx storage reduction catalyst.
 4. The exhaust gas cleaning device according to claim 3, wherein, when the detected temperature from the temperature detection unit is lower than a reaction enabling temperature of the catalyst, the control unit performs temperature raising control of the internal combustion engine so that the detected temperature from the temperature detection unit becomes equal to the reaction enabling temperature.
 5. The exhaust gas cleaning device according to claim 3, wherein the added amount of the reducing agent is higher in the second reducing agent addition than in the first reducing agent addition. 